From a Future History "Timeline" I am making that I did post some other stuff out of before, but these are the first uniform/rank insignia stuff out of it.
Uniforms and Rank Insignia of the Legions of Hermes, the military of the State of Hermes, which ruled over Mercury from 2681 (taking over the Republic of Mercury via self-coup of the President against the the legislature and the other parties with military force) to 2841 (Venusian conquest of Mercury). The Legions/State of Hermes was not the only example of "planetary legions" through history, but it was one which was the most Roman. The earlier "Legions" had stopped with the use of that title as a political party and not much else, having governments and militaries not much different from other contemporary states. Similarly the later Legions of Zeus and Chronos were simply among the largest of Jovian and Saturnian independence organisations against Venusian Empire, and were not connected to "Legionary Ideologies".
The State of Hermes was an exception, as it's military was highly based on the Roman one. Said military was built for and saw action in the Hermetic War, 2687-2707. Where they saw combat all across the Solar System, from Venus to Saturn. They were defeated in the attempt but did great damage to the Solar System, causing the start of the one and a half century time period known as the Interregnum, until humanity was united by the Venusian Empire and the Pax Venusia started
Firstly the dress uniforms for enlisted personnel. Of the Legions proper/ground forces, of the Reserves, of the Auxilia and of the Navy (space warfare branch) respectively. Uniforms are presented without unit or rank insignia, as well as the pugio daggers that are officially a part of the dress uniform. However the colouring of the collars and the pillbox hat does show that the Ground Forces Legionary is of the elite Legio XIII "Imperator's Own", the Reserve Legionary is of the 1st Reserves stationed on Mercury itself, the Auxiliary is from Earth, an orbital habitat in it's orbit or Luna, while the Fleet Legionary is in the Home Fleet.
The rank insignia would be on their upper right arm, while Unit (Legion, Reserve Regiment, Auxilia Cohort or Navy Ship/Squadron) insignia would be on the upper left arm. Moreover the number of the Cohort (in Roman numerals) and the number of the Century (in Arabic numerals) would be on the collars of all but the Navy soldier, for them it would show their department on the ship (Weapons, Engineering etc.) they belonged to.
An officer's dress uniform would have the following differences: instead of the pugio dagger they would have a gladius sword (with the exception of Auxilia officers, who would be issued with longer spatha swords), officers wore helmets based on that of the Roman Empire with coloured crests (same colours as with the enlisted pillbox hats). On some ceremonial occasions and parades the officers were also issued lorica segmentata armour.
The service uniform for all services but the Navy is the same as the dress uniform, minus the dagger/sword and any armour. In this uniform enlisted wear berets of the same colour as their dress uniform pillbox hat while the officers wear peaked caps (much like how the Imperator is depicted below, with the colouring of the band matching the colur of the officer's helmet crest. The Navy service dress on the other hand is a single piece gloved flightsuit, able to be sealed airtight with a helmet. The flightsuit is colured blue, with some piping in grey/purple/black according to the which part of the fleet they are under.
There is also the field/combat uniform, which comes with one of five camouflage patterns. These are: Type A, or Urban, Shipborne and Moon Camouflage, in various shades of grey, the most common camo issued, having been issued to essentially all units except for those headed for Eath, Venus and Mars; Type B, or Temperate, Forest and Jungle Camouflage, in green and brown shades, issued for the invasion of Earth Venus and Mars; Type C, or Polar/Ice Camouflage, issued for Earth, Venus, Mars and the Galilean Moons (yeah, shame about Io being mostly Sulphur yellow with no camouflage for it); Type D, or Primary Desert Camouflage, in tan and brown colours, issued mainly for Earth and finally Type E, or Secondary Desert Camouflage, in reddish and orange colours, issued mainly for Mars, also issued for Titan. Field Uniforms both come in actual battle dress to use in places with breathable athmosphere and an armoured pressure suits/space suits.
This shows various unit colours. To make a list:
1st Reserves - Purple (I to X Cohort of the Reserves, stationed on Mercury, as a side note, Mercury was originally colonised by the United States of America)
2nd Reserves - Red hat, Blue collar (XI Cohort of the Reserves, garrisoning space habitats on Sun-Mercury L4, originally colonised by the Unified Commonwealth, which the colours honour)
3rd Reserves - Light Blue (XII Cohort of the Reserves, garrisoning space habitats on Sun-Mercury L5, originally colonised by the UN, which the colour references)
Orbital Defence Command - Grey (orbital defense ships and stations orbiting Mercury and the lagrange point colonies, when the Home Fleet was deployed as reinforcements to Venus the crew of the Corvettes, which were left behind as they lacked the Delta-V, switched from their Home Fleet uniforms to Orbital Defence uniforms)
Home Fleet - Purple (kept these uniforms, even as the Fleet was deployed to Venus)
Expeditionary Forces - Black (majority of the Navy, referring to the Black of Space)
Aphroditian Auxilia - Copper (due to the planet's association with the metal)
Gaian Auxilia - Light Blue (referencing the UN colour, covers Luna, traditionally considered a separate Legionary "Nation" as the State of Artemis)
Aresian Auxilia - Red (Mars is the Red Planet, obviously)
Belt Auxilia - Gren hat and grey collars (the collar references the Asteroid Belt in general, while the hat references Demeter, symbolising Ceres, the most important asteroid)
Zeusian Auxilia - Purple (in reference to Jupiter/Zeus as King of the Gods)
Chronosian Auxilia - Orange (referencing the colour of Saturn and Titan)
Uranian Auxilia - Blue (never worn as Hermetic forces never made it to Uranus, colur chosen due to the colour of Uranus and the association with the Sky)
Poseidonian Auxilia - Sea Green (never worn as Hermetic forces never made it to Neptune, colour chosen due to Neptune's association with the Sea)
Hadesian Auxilia - Black (never worn as Hermetic forces never made it to Pluto, intended to cover Auxilia from all Trans-Neptunian objects)
Legio I - Maroon (1st Legion was an elite formation, it used these colours in reference to airborne forces)
Legio II - Grey (considered the colour of Mercury/Hermes)
Legio III - Olive (general military colours)
Legio IV - Light Blue (4th Legion was assigned to invade Earth)
Legio V - Silver (5th Legion was assigned to invade Luna)
Legio VI - Copper (6th Legion was assigned to invade Venus)
Legio VII - Rifle Green (titled "Lucky" it was an elite formation, thougt said title was not the result of any experience)
Legio VIII - Blue (the "Amphibious" Legion, another relatively elite formation)
Legio IX - Red (9th Legion was assigned to invade Mars)
Legio X - Black (the "Cavalry" Legion, the 10th had more armoured vehicles than the usual Legion, following British armoured tradition)
Legio XI - Brown
Legio XII - Dark Green ("Fulminata", the Thunderbolt Legion was another elite formation, referencing commando colours)
Legio XIII - Purple ("Imperator's Own", relatively obvious)
Legio XIV - Green hat, grey collar (14th Legion was assigned to take over the Belt, the grey referring to the Asteroids in general, the green referring to Ceres in specific)
Legio XV - White (assigned to invade the Jovian system, referencing the Icy Galilean Moons)
Legio XVI - Yellow hat, purple collar (the hat references the Galilean moon of Io, the collar tabs Jupiter)
Legio XVII - Orange (intended to invade the Saturnian system)
Legio XVIII - Tan (intended to invade Uranus, the colour is general military tradition, some OTL units have tan berets)
Legio XIX - Sea Green (intended to invade Neptune, and then send detachments to invade the Trans-Neptunian bodies)
The purple tunic with the peaked cap depicts the service uniform of the Imperator, the head of state and commander in chief of the armed forces, thought without the rank insignia. Red and Blue collar tabs show his command of the ground forces and the fleet.
Finally, the rank insignia:
Regular insignia at top (given in ground forces colours, the Navy and Reserves have the same ranks and insignia, thought the colouring would be different), Auxilia insignia at the bottom.
The ranks being, left to right:
Legionary, Senior Legionary, Decanus, Decurion, Tesserarius, Optio, Signifer, Centurion, Senior Centurion, Prefect, Tribune, Legate, Praetor, Imperator for the Legions proper.
Auxiliary Recruit, Auxiliary First Class, Decanus, Decurion, Tesserarius, Optio, Signifer, Centurion, Prefect and Tribune for the Auxilia.
Now you'll notice the Auxilia rank table has three holes in it. Firstly, an Auxiliary Recruit has no rank insignia (as with privates in many armies OTL), secondly the Auxilia has no Senior Centurion rank. Auxiliary Cohorts tend to be commanded by the Centurions of their first Centuries (as with the Legions, except there these officers are ranked Senior Centurion) or by Prefects. As for the holes at the top ranks, Tribune is the highest Auxilia rank and is more of an appointment than rank anyway. As an Auxiliary Tribune is the commander of an Auxilia Legion, which is a relatively rare and ad-hoc formation (Auxilia Cohorts are mostly attached to Hermetic Legions).
(Highest ranked Reserve officer would be the Legate of the Reserves, as the professional head of the entire Reserve, while there would also be a single Tribune of the Reserves in command of the 1st Reserves, while there would also be 7 Prefects, two of which would be in command of the 2nd and 3rd Reserves. Otherwise they would follow the Legionary ranks.)
(In contrast the Navy would be a rather top heavy organisation. With there being more of the Naval top rankers than ground forces, what with operating spaceships being rather complex. Probably more NCOs than soldiers ranked Legionary and Senior Legionary on board actual ships.)
Regarding the rank insignia, I was originally planning to replicate the US Army Staff Sergeant, Sergeant First Class and Master Sergeant for Legionary Decurion, Tesserarius and Optio. However I had difficulty trying to make that arc properly in MS Paint and ended up really liking the current Decurion insignia when I made it. Then the Tesserarius and Optio insignia followed naturally from it both by US inspiration and the already made insignia for the Auxilia. For the officer insignia the with junior officers I followed British style pips and the for the senior officers I followed the US general insignia.
Now to give explanation for the ranks themselves. Legionary/Auxiliary Recruit corresponds to NATO OR 1 and 2. Senior Legionary/Auxiliary First Class being an OR-3. Decanus was the rank squad/contubernium leaders in the Roman Legions had, and is equilavent to a modern Corporal. Decurion here is equilavent to a sergeant (OR-5/6), in Rome they commanded 30 man cavalry units named Turma. I am using that unit name for platoons in the Hermetic Legions. There was one Tesserarius per Century, and the rank translated as the officer of the Watch. I am using it for OR-7/8. Optio is the highest rank of NCO. There is one per Century, where they are the senior NCO and main advisor to the Centurion.
For the officer ranks Signifer was actually the the title for the soldiers carrying the standard of the Century. The Aquilifer carrying the Legion's Aquila was therefore a type of Signifer. Considering that carrying the regimental standard is commonly the duty of junior officers in modern militaries (as it is where the rank of Ensign/Cornet originated from, which later became Second Lieutenant) I felt that it was a decent choice for a Lieutenant (OF-1) rank. In the State of Hermes, they command Turmae/Turmas (Platoons). One rank above is that of Centurion aka Captain/Company Commander. Next is the Senior Centurion (historical Primus Pilus/Pilus Prior), which is the rank for most Cohort commanders (who also command the 1st Century of their Cohorts). In the Navy commanders of auxiliary ships (ammunition ships, fuel tankers, troop carriers etc.) are often of this rank. Prefect is the above it, who mostly serve as administrative/staff officers. Thought they also often serve as commanders of Reserve and Auxilia Cohorts. In the Navy Destroyers, Frigates and Corvettes are often commanded by them (thought some Corvette commanders might be Senior Centurions instead). Thought I used Prefect this rank is more similar to narrow band/lower ranked Tribunes (other than the command of Auxilia Cohorts thing, that is a Prefect thing). The rank above this is Tribune, the second in command of a Legion. Here I also used it for the commanders of Legions of lesser prestige/station compared proper ones (1st Reserve and the Auxilia Legions). Naval officers with this rank command Destroyer/Frigate Squadrons as well as warships cruiser or larger. Then there is the largest regular rank, which is Legate (equilavent to OF-6). Who are commanders of Legions and Cruiser Squadrons (Hermetic Battleships and Carriers did not usually operate in squadrons). Thought you'll notice there is a rank between this and the Imperator. And that is the rank of Praetor (covering OF 7, 8 and 9). This is more of an appointment than a rank. It is specifically granted to the Legate of one of several legions operating together. Giving them clear authority over other Legates. In the Navy Praetors commanded several Taskforces and Taskgroups under the Expeditionary Forces, as well as the Home Fleet. While it was officially a single rank Praetors expected to operate together (such as Army and Navy Praetors or multiple Praetors in a single fleet) the orders giving them their mission and appointment as Praetor will also include a list of which Praetor is considered superior to which. Thought as the Hermetic War progressed and higher officers had many losses the uncertainties in the chain of command between multiple legions expected to work together or between the Legions and Fleet did indeed cause many issues.